Releasable sheets are roughly classified based on the composition of the substrate, into releasable sheets prepared from petroleum-derived synthetic polymers such as polyesters and polyolefins and releasable sheets prepared from various paper substrates such as glassine paper and clay coated paper. Releasable sheets are used for the purpose of protecting a pressure-sensitive adhesive surface or other material surfaces. Once the releasable sheets are no longer required after use, they are usually discarded. However, with environmental awareness growing in recent years, a petroleum-derived synthetic polymer film is sometimes regarded as imposing a high impact on the environment. On the other hand, a paper substrate subjected to a predetermined treatment may be recycled as wastepaper. From the standpoint of transparency, smoothness, dust resistance and the like, however, paper tends to be inferior to a releasable sheet formed from a synthetic polymer film.
Polylactic acid (PLA) is receiving significant consideration as an environmentally friendly resin. PLA is a biomass-derived resin that is classified as a renewable resource because it is made from plants such as corn or sugarcane. PLA requires a relatively small amount of energy for production and typically generates a relatively small amount of carbon dioxide per unit weight when incinerated.
However, PLA films tend to have poor heat resistance when compared to general-purpose aromatic polyester films. Therefore, unlike a polyester film, PLA-based films cannot be readily heat-cured (for example at 140 to 160° C.) after a reactive silicone has been coated thereon. For example, as described in Kokai (Japanese Unexamined Patent Publication) No. 2000-280429, even reactive silicones that can be cured at relatively low temperatures (for example 100° C.) can cause deformation of a PLA film. The thermal damage to the PLA film could be reduced by shortening the cure time. However, shorter cure times require the use of large amounts of expensive catalysts. Such catalysts would likely render the PLA releasable sheets too expensive for regular use. The short time and/or low temperatures of such curing can also leave the silicone partially uncured which could allow it to migrate into the pressure-sensitive adhesive surface and impair the residual adhesive force of the pressure-sensitive adhesive.
Because of the drawbacks noted above, there remains a need for new environmentally responsible releaseable sheets that still provide acceptable properties.